The present invention in general relates to a method of surface treatment using electric discharge and an electrode used to obtain the electric discharge. More particularly, this invention relates to a method of surface treatment using electric discharge in which a gap discharge is carried out in a fluid.
A method of surface treatment using electric discharge (hereafter, discharge surface treatment method) is known. In the conventional method, a discharge in the form of pulses is generated between an electrode (hereafter, discharging electrode) and a work. The discharging electrode and the work are aligned face to face with a predetermined gap (hereafter, discharging gap) in a machining fluid. The discharging electrode is a green compact electrode or a metal electrode. The green compact electrode is the one that is formed by compressing metal powder or powder of a metal compound or ceramics powder. A hard coat film made of the material with which the electrode is formed (hereafter, electrode material) or its reactant is formed on the surface of the work (hereafter, work surface) because of the energy generated due to the electric discharge. This conventional method is disclosed in Japanese Patent Application Laid-Open gazettes (JP-A No. 8-300227, JP-A No. 9-19829 and JP-A No. 9-192937).
In conventional discharge surface treatment methods, a formed electrode is not used, but a solid discharging electrode having a simple shape that can be easily produced, such as a round rod-shaped electrode. The discharging electrode is made to scan the work surface.
However, when a discharging electrode having a shape of a round rod is used, as illustrated in FIG. 4(a), the length L of the electrode surface in the scanning direction is the longest in the electrode center portion and it is gradually shortened toward the sides. Therefore, when the discharging electrode passes by during scanning, an area through which the center portion passes has a longer time to face the discharging electrode, while another area through which a side portion passes has a shorter time to face the discharging electrode. For this reason, as illustrated in FIG. 4(b), the resulting coat film becomes thicker at the area through which the electrode center portion with the long length L of the electrode face passes, while it becomes thinner at the area through which the electrode side edge portion with a short length L of the electrode face passes, with the result that the electrode passage of only one time forms a levee shaped coat film c with variations in the film thickness, failing to form a coat film face with a uniform thickness.
When a coat film is formed along a straight line portion, a discharging electrode of a prismatic rod having a square shape in its cross-section maybe used. However, when, upon application of the prismatic rod discharging electrode, the scanning direction of the discharging electrode is changed, the orientation of the discharging electrode has to be rotated as in the case of spring processes, resulting in a difficult and time-consuming task.
Moreover, in the case of the discharging electrode of a round rod, upon carrying out the process on an edge portion of a mold, etc., the end face of the electrode is consumed as the process proceeds, with the result that, as illustrated in FIG. 5, the edge portion e of a work W is surrounded by the end face A of the electrode in such a manner that discharging is concentrated on the edge portion e, causing round edge corner portions with edge sagging.
In order to solve this problem, a method has been proposed in which the discharging electrode is rotated around the center axis line thereof; however, even if the discharging electrode is rotated, it is not possible to solve the problem of variations in the coat film thickness depending on the positions through which the discharging electrode passes.
Here, in a discharge coating method in which the tip of an electrode is allowed to slightly contact a work and the electrode material is transferred and fused to coat the work little by little through the discharging energy, a hollow pipe electrode is used as described in Japanese Patent Application Laid-Open gazettes (JP-A No. 8-53777 and JP-A No. 1-139774).
The present invention has been devised to solve the above-mentioned problems. It is an object of this invention to provide a discharge surface treatment method which can form a coat film with a uniform thickness in the discharge surface treatment method by using the gap-discharging process in a fluid, and which can also form a coat film without causing edge sagging, and also to provide a discharge surface treatment electrode used for such a discharge surface treatment method.
The method of the present invention, which is a discharge surface treatment method in which an electric discharge in the form of pulses is generated between an electrode and a work, the electrode and the work being aligned face to face with a predetermined gap therebetween in a machining fluid, the electrode being a green compact electrode formed by compressing metal powder or powder of a metal compound or ceramics powder, or a metal electrode, so that a hard coat film of the material with which the electrode is made or a reactant of this material is formed on a surface of the work because of the energy generated during the electric discharge, wherein the electrode is cylindrical in shape.
Because the electrode is cylindrical in shape, the length of the electrode face in the scanning direction is made virtually equal to each other between the electrode center portion and the electrode side edge portions. Accordingly, a coat film having virtually the same thickness at the electrode center portion and at the electrode side edge portions is formed so that a coat film surface having a uniform thickness can be formed. Moreover, since the electrode has cylindrical shape, different from a square shape, it does not have any directivity with respect to the scanning direction (shifting direction) Thus, it is not necessary to rotate the direction of the discharging electrode even when the scanning direction of the discharging electrode is changed.
Moreover, the present invention makes it possible to provide a discharge surface treatment method for forming a hard coat film while rotating the discharging electrode having the cylinder shape around its center axis line.
Therefore, the rotation of the discharging electrode around its center axis line allows the discharging electrode to be consumed in a uniform manner, thereby providing a uniform coat film thickness as well as preventing sagging at edge portions.
The electrode of the present invention, which electrode is used in a discharge surface treatment method in which an electric discharge in the form of pulses is generated between an electrode and a work, the electrode and the work being aligned face to face with a predetermined gap therebetween in a machining fluid, the electrode being a green compact electrode formed by compressing metal powder or powder of a metal compound or ceramics powder, or a metal electrode, so that a hard coat film of the material with which the electrode is made or a reactant of this material is formed on a surface of the work because of the energy generated during the electric discharge, wherein the electrode is cylindrical in shape.
Therefore, the length of the electrode face in the scanning direction is made virtually equal to each other between the electrode center portion and the electrode side edge portions; thus, a coat film having virtually the same thickness at the electrode center portion and at the electrode side edge portions is formed so that a coat film surface having a uniform thickness can be formed. Moreover, this discharging electrode has a cylinder shape so that, different from a square shape, it does not have any directivity with respect to the scanning direction (shifting direction); thus, it is not necessary to rotate the direction of the discharging electrode even when the scanning direction of the discharging electrode is changed.